Method and device for stabilizing a vehicle combination

ABSTRACT

Method and Device for Stabilizing a Car-Trailer Combination A method and device for stabilizing a car-trailer combination, including a towing vehicle and a trailer moved by the towing vehicle, is disclosed. The rolling motions of the towing vehicle are monitored and measures that stabilize driving are preformed upon detection of an actual or expected unstable driving condition of the towing vehicle or the car-trailer combination. In order to insure a proper intervention, the yaw velocity of the towing vehicle or the car-trailer combination is detected and the measures that stabilize driving conditions are controlled dependent upon the detected yaw velocity.

TECHNICAL FIELD

The present invention relates to a method and a device for stabilizing acar-trailer combination, including a towing vehicle and a trailer movedby the towing vehicle, wherein the towing vehicle is monitored in termsof rolling motions and measures that stabilize driving are taken uponthe detection of an actual or expected unstable driving performance ofthe towing vehicle or the car-trailer combination.

BACKGROUND OF THE INVENTION

The present method aims at detecting and controlling the instabilitiesof car-trailer combinations (motor vehicle with trailer), especially ofcombinations consisting of a passenger car, pickup truck orsport-utility vehicle and any trailers desired, in particular caravans,before driving conditions are encountered during which the driver can nolonger maintain control of the vehicle. These unstable conditionsinvolve the rolling motions known with car-trailer combinations and theanti-phase build-up process between the towing vehicle and the traileras well as imminent roll-over conditions at too high lateralaccelerations caused by obstacle avoidance maneuvers, lane changes, sidewind, road irregularities and/or hasty steering maneuver requests by thedriver.

Depending on the driving speed, the oscillations can decay, remainconstant, or increase (undamped oscillation). When the oscillationsremain constant, the car-trailer combination has reached the criticalvelocity. Above this speed threshold a car-trailer combination isunstable, below said threshold it is stable, that means, possibleoscillations die out.

The magnitude of this critical speed depends on the geometry data, thetire rigidities, the weight and the distribution of weight of the towingvehicle and the trailer. Further, the critical speed is lower in abraked driving maneuver than at constant travel. In turn, it is higherduring accelerated driving than at constant travel.

Corresponding methods and devices are known in various designs (DE 19953 413 A1, DE 199 13 342 A1, DE 197 42 707 A1, DE 100 34 222 A1, DE 19964 048 A1).

DE 197 42 707 C2 discloses a device for damping rolling motions for atleast one trailer towed by a towing vehicle, wherein the angularvelocity of the trailer about the instantaneous center of rotation orthe articulated angle about the instantaneous center of rotation issensed and differentiated and taken into consideration for controllingthe wheel brakes of the trailer. Acceleration sensors at differentlocations are used as sensors for the angular velocity. DE 199 64 048 A1also provides a lateral acceleration sensor or a yaw rate sensor bymeans of which the rolling motion is determined. After the signal isevaluated, a periodic yawing torque shall be applied to the vehicle. DE100 34 222 A1 determines a time for a braking intervention correct inphase, being realized in dependence on the quantity of frequency and thephase magnitude of the rolling motion.

In addition, it is known from EP 0 765 787 B1 to take measures thatdecelerate driving when the amplitude of a vehicle quantity related totransverse dynamics and swinging within a frequency range exceeds apredetermined limit value and when a steering motion quantity does notexceed a predetermined threshold. In this case, likewise the lateralacceleration and/or the yaw velocity (yaw rate) is taken intoconsideration as a vehicle quantity measured on the vehicle.

In doing so, it is necessary to monitor the steering angle with respectto a predetermined threshold in order to take the measures thatdecelerate the vehicle only when the steering angle is as constant aspossible.

Hence, the stabilization strategy of all design variants can besummarized as follows:

-   -   Detection of the rolling motion by evaluating the sensor data,        with all sensors being favorably accommodated in the towing        vehicle or the trailer;    -   When an unstable situation is detected, the vehicle is slowed        down by reducing the engine torque and building up pressure in        the wheel brakes of the towing vehicle;    -   Additionally or alternatively a torque about the vertical axis        of the towing vehicle is applied, said torque counteracting the        force transmitted from the trailer to the towing vehicle and,        thus, damping the oscillation.

The detection of the rolling motion of a car-trailer combination ismainly based on the fact that the yaw rate or lateral acceleration showsan almost sinusoidal variation, the frequency of which lies in a typicalband, without the driver performing corresponding steering movementsthat would cause the observed variation of lateral quantities. It isproblematic with this detection strategy that there are still othermaneuvers generating similar variations of signals. Thus, sprung-massvibrations may be produced, e.g. during cornering with a constantsteering angle, which also induce sinusoidal variations of lateralquantities. Another possibility of obtaining such variations of lateralquantities is to drive over rough roadways, in particular wavy roadsections, especially bumps on alternating sides.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method and a devicepermitting the reliable detection of unstable driving performance.

According to the invention, this object is achieved in that the yawvelocity is detected and measures that stabilize driving are controlleddependent on a differential value that is produced from the detected yawvelocity and a model-based yaw velocity and evaluated according tocriteria indicative of an unstable driving performance.

Advantageously, the method allows for reliably detecting snakingcar-trailer combinations, in particular passenger car/trailercombinations. In this method, a differential value Δ{dot over (ψ)} isgenerated from the measured yaw rate and the model-based reference yawrate, which value is representative of the deviation of the vehicle fromthe track predetermined by the steering wheel position. Because thisdifferential value represents only the deviation from the desired track,monitoring the differential value ensures the judgment of oscillationsindependently of a curved track passed e.g. due to a steering angle.Preferably, the differential value is filtered in a low-pass filter inorder to cut off signal peaks triggered by the detection of coefficientsof friction. Spurious detections and, thus, faulty control activationsare avoided in addition. The method and the device favorably requireonly a sensor system provided in an Electronic Stability Program (ESP)driving stability control.

In this arrangement, an actuating signal for an electric motor of ahydraulic pump producing a brake pressure and, hence, actuating thewheel brake of the towing vehicle or the trailer is generated by way ofthe data measured by a yaw rate sensor, derived in an ESP drivingdynamics control operation and logically combined with the ESP controlstrategy. In this data the data of a motor vehicle can be included. Itis possible alternatively or additionally to drive an actuator of anoverriding steering system. By applying equal or different brakepressure to one wheel of preferably the towing vehicle or to all wheelsof the towing vehicle corresponding to an ESP control strategy, it ispossible to correct the instabilities of the car-trailer combinationdetected by sensors and to reduce the possibly existing excessivetransverse dynamics of the car-trailer combination by reducing thevehicle speed and/or the lateral forces at one wheel by means ofincreased brake pressure and/or the increase in the longitudinal forces.

It is favorable that the frequency and the amplitude of each half waveof the differential value is determined, compared to stored values, andthe rolling motion of the car-trailer combination is evaluated independence on the result of the comparison.

Advantageously, the oscillation frequency of the car-trailer combinationis achieved in that the frequency is determined from the zero crossingsand the time between two zero crossings of the yaw velocity.

The condition for detecting a snaking, unstable car-trailer combinationis favorably satisfied by the following steps: counting the number ofthe half waves of the differential value where the amplitude of eachhalf wave reaches or exceeds a threshold value, counting each positiveand negative half wave of the determined frequency when each positiveand negative half wave lies within a band defined by a top and a bottomthreshold value, and comparing the value of the half waves counted witha threshold value representative of a number of half waves, and measuresthat stabilize driving are initiated when the threshold value is reachedor exceeded. It is favorably arranged so that the conditions arecontinuously satisfied and the half waves are serially counted in orderthat the threshold value representative of a number of half waves isreached or exceeded, respectively. The threshold value representative ofa number of half waves can favorably be determined in dependence on thefrequency, and at low frequencies, the threshold value is reached orexceeded with a smaller number of half waves than is the case at a highfrequency.

Further, it is advantageous that the threshold value of each half waverepresentative of the amplitude is determined at least in dependence onquantities that represent the velocity of the towing vehicle or thecar-trailer combination or the trailer. It is arranged for that withquantities describing a high speed, the threshold value is reached orexceeded at lower amplitudes than is the case with quantities describinga low speed.

To avoid constant activation and deactivation of the controller (ESPdriving stability controller), only a consecutive number of half wavesof the yaw velocity are counted where the amplitude of each half wavereaches or exceeds an entry threshold value, and the measures thatstabilize driving are terminated when values reach or fall below onlyone exit threshold value ranging below the entry threshold value.

In a preferred embodiment of the invention, the data is produced fromthe variation of the differential value. The model-based yaw velocity iscalculated in a vehicle model that is a component of an ESP drivingstability control in a favorable manner. In the vehicle model, inparticular the single-track model, the model yaw rate is generallyproduced from the steering angle, the lateral acceleration and thevehicle speed (vehicle reference speed).

Surprisingly, it has been shown that during rapid changes of thesteering angle, i.e. at high steering angle speeds, deviations in thevehicle model are generated which cause a signal variation that isconfusable with the monitored signal variation when the car-trailercombination is snaking. It is assumed that these deviations are due tothe reaction times of the signal generation, on the one hand, and theretarded vehicle reaction, on the other hand. To avoid these faultydetections, provisions are made to ensure that the differential value isweighted with a value, in particular a factor, which is produced independence on the steering angle velocity or the steering angleacceleration or preferably the model deviation or deviation of thereference yaw rate, respectively. The reason is that it has been foundout that the model yaw rate deviation or the model yaw rate speed,respectively, is most appropriate for filtering the differential valuebecause the vehicle speed vRef and the steering angle velocity {dot over(δ)} go into said value.

In a particularly favorable embodiment of the method, the lateralacceleration is detected and the variation of the lateral accelerationis evaluated according to criteria which allow checking the plausibilityof the data obtained from the variation of the differential value andbeing assessed according to criteria indicative of an unstable drivingperformance.

Plausibility is checked by way of finding out the maximum and minimumvalues of the lateral acceleration and their temporal distances, bydetermining the frequency and comparing it with the frequency of thedifferential value.

Plausibility is additionally checked and the method is terminated or themeasures that stabilize driving are discontinued when at least one ofthe following conditions is satisfied:

-   -   the frequency of a lateral signal or a transverse quantity, such        as the lateral acceleration and/or the differential value        reaches or exceeds or, respectively, falls below a top or a        bottom threshold value;    -   the frequency of the lateral signal changes in relation to the        frequency of the differential value towards a top or a bottom        limit value;    -   the absolute value of the average value of the lateral signal        exceeds a threshold value;    -   the amplitude of the lateral signal decreases with a high        gradient; and/or    -   the difference between the maximum and minimum values of the        lateral signal lies in a narrow band.

As the phase shift is small in snaking car-trailer combinations, it isfavorably arranged so that the phase shift between the lateralacceleration and the differential value is determined and evaluatedaccording to criteria that permit determining driving situations.

It is favorable that the measures that stabilize driving arediscontinued or the method is terminated, respectively, when a thresholdvalue indicative of a great phase shift is exceeded.

Further, an object of the invention relates to a device for stabilizinga car-trailer combination, including an ESP driving stability control,with a yaw rate sensor for sensing the yaw velocity and a vehicle modelfor producing a reference yaw velocity, with a determining unitdetermining a differential value from the yaw velocity and the referenceyaw velocity, with a control unit controlling measures that stabilizedriving in dependence on data being obtained from the variation of thedifferential value and evaluated according to criteria indicative of anunstable driving performance.

An embodiment of the invention is illustrated in the accompanyingdrawings and described in more detail in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 shows a vehicle with an ESP control system.

FIG. 2 shows the variation of signals of the differential value of thesnaking towing vehicle.

FIG. 3 shows the signals of a snaking towing vehicle.

FIG. 4 is a simplified flow chart showing the control.

FIG. 5 is a simplified wiring diagram for calculating the differentialvalue Δ{dot over (ψ)}.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the actual method is referred to, FIG. 3 shall be used toschematically explain the signal variation of the oscillation of the yawrate (dash-dot), of the steering angle (dash-dash), and the differentialvalue of measured yaw rate and model or reference yaw rate in dependenceon a slalom maneuver or the slalom-like avoidance of obstacles,respectively. The signal variation a) shows a sinusoidal variation ofthe yaw rate {dot over (ψ)} and the differential value Δ{dot over (ψ)}of model yaw rate and measured yaw rate without the driver steering.Without a corresponding steering angle variation, the variation of theyaw rate and the differential value of measured yaw rate and model-basedyaw rate are almost equal.

FIG. 3 b) shows the signal variation that is e.g. produced in a slalommaneuver when the oscillation is generated by the steering anglevariation alone, where the vehicle can follow the driving performance ofthe driver illustrated in the vehicle model. In this case, thedifferential value at issue is zero because no deviation betweenmeasured yaw rate and model-based yaw rate is determined. The vehiclefollows the steering angle predefined by the driver.

FIG. 3 c) shows the signal variation in dynamic slalom maneuvers. Hereinthe oscillation is generated alone by the steering angle variation dueto the rapid steering angle changes, i.e. at high steering anglevelocities. The sinusoidal variation of the differential value isgenerally based on the fact that the vehicle can no longer follow thevehicle model. That means, the model yaw rate determined in the vehiclemodel is no longer identical with the measured yaw rate because thevehicle is no longer able to instantaneously comply with the dynamicsteering angle variations.

FIG. 1 shows a vehicle with an ESP control system, brake system, sensorsystem, and communication provisions. The four wheels have been assignedreference numerals 15, 16, 20, 21. One wheel sensor 22, 23, 24, 25 isprovided at each of the wheels 15, 16, 20, 21. The signals are sent toan electronic control unit 28 determining from the wheel rotationalspeeds the vehicle speed v by way of predetermined criteria. Further, ayaw rate sensor 26, a lateral acceleration sensor 27, and a steeringangle sensor 29 are connected to the electronic control unit 28.Further, each wheel includes an individually actuatable wheel brake 30,31, 32, 33. The brakes are hydraulically operated and receivepressurized hydraulic fluid by way of hydraulic lines 34, 35, 36, 37.The brake pressure is adjusted by way of a valve block 38, said valveblock being actuated irrespective of the driver by way of electricsignals produced in the electronic control unit 28. The driver canintroduce brake pressure into the hydraulic lines by way of a mastercylinder actuated by a brake pedal. Pressure sensors P are used to sensethe driver's braking request are provided in the master cylinder or thehydraulic lines, respectively. The electronic control unit is connectedto the engine control device by way of an interface (CAN).

It is possible to provide a statement about the respective drivingsituation and, thus, to realize an activated or deactivated controlsituation by way of a determination of the entry and exit conditions bymeans of the ESP control system with brake system, sensor system, andcommunication provisions that includes the following pieces ofequipment:

-   -   Four wheel speed sensors    -   pressure sensor (brake pressure in master cylinder p_(main))    -   Lateral acceleration sensor (lateral acceleration signal        a_(actual), lateral inclination angle α)    -   Yaw rate sensor ({dot over (Ψ)})    -   Steering wheel angle sensor (steering angle δ, steering angle        velocity {dot over (δ)})    -   Individually controllable wheel brakes    -   Hydraulic unit (HCU)    -   Electronic control unit (ECU).

This renders possible one main component of the method for stabilizingcar-trailer combinations, i.e. the detection of driving situations,while the other main component, i.e. the interaction with the brakingsystem, also makes use of the essential components of the drivingstability control.

A conventional ESP intervention is used to produce an additional torqueby purposeful interventions at the individual brakes of a vehicle, saidtorque adapting the actually measured yaw angle variation per unit oftime (actual yaw rate {dot over (Ψ)}_(actual)) of a vehicle to the yawangle variation per unit of time (reference or model or nominal yaw rate{dot over (Ψ)}_(no min al), respectively) influenced by the driver. Inthis arrangement, the input quantities which result from the trackdesired by the driver are sent to a vehicle model circuit which, by wayof the prior-art single track model or any other driving model,determines a model yaw rate ({dot over (Ψ)}_(no min al)) from theseinput quantities and from parameters being characteristic of the drivingperformance of the vehicle, but also from quantities predefined bydistinctive features of the ambience. Said model yaw rate is compared tothe measured actual yaw rate ({dot over (Ψ)}_(actual)). The differencebetween the model yaw rate and the actual yaw rate (Δ{dot over (Ψ)}) isconverted by means of a so-called yaw torque controller into anadditional yaw torque M_(G) which represents the input quantity of adistribution logic.

Distribution logic, in turn, determines the brake pressure to be appliedto the individual brakes, possibly in dependence on a braking request ofthe driver demanding a defined brake pressure at the wheel brakes. Thepurpose of the brake pressure is to produce an additional torque at thevehicle in addition to the desired brake effect, as the case may be,said torque supporting the driving performance of the vehicle in thedirection of the steering request of the driver.

FIG. 5 schematically shows that part of the ECU 28 wherein thedifferential value Δ{dot over (ψ)} is calculated. ECU 28 includes avehicle model 50 for producing a model yaw rate. At least the steeringangle and the vehicle speed vRef are sent to the vehicle model 50.Further data, which can be included in the model, are the lateralacceleration, the measured yaw rate and a coefficient of frictiondetermined in a coefficient-of-friction and situation detection unit.The model yaw rate is produced from the input signals in the model. Inthe determining unit 51, the model yaw rate is compared with the yawrate sensed by the yaw rate sensor 26, and the differential value isdetermined from the yaw rate and the model yaw rate. The differentialvalue Δ{dot over (ψ)}/dt is weighted by a factor produced in dependenceon the model yaw rate change and is filtered in filter 52. The factor ≠0prevents the spurious detection that has been described with respect toFIG. 3 c).

FIG. 2 exhibits the signal variation of the differential value of asnaking towing vehicle.

As a first component of the detection, the method comprises a module foranalyzing the variation of the difference of the model/actual yaw rateΔ{dot over (ψ)}. The model detects zero crossings 60, 61 of thedifferential value between the model yaw rate and the measured yaw rate,said differential value to be taken into account for the analysis, anddetermines the time between two zero crossings. The oscillationfrequency is thereby obtained. A half wave is recognized as valid onlyif the determined frequency lies within a typical band (roughly 0.5-1.5hertz). Further, a half wave is valid only if the amplitude between twozero crossings has exceeded a defined threshold. The number of the validhalf waves is counted. When the number of the valid half waves exceeds athreshold value, the differential value condition for detecting asnaking car-trailer combination is satisfied.

Steering movements of the driver are considered directly in thedetection signal by way of monitoring the difference between the modelyaw rate and the measured yaw rate. When the driver e.g. carries out aslalom maneuver at a low vehicle speed with a low steering anglevelocity, admittedly, the measured yaw rate shows a variation from whicha snaking car-trailer combination could be concluded. However, the modelyaw rate shows the same variation in the slalom maneuver so that thedifference signal is almost zero and a spurious detection is ruled out.Thus, spurious detections caused by slalom maneuvers are thus avoideddue to this embodiment of the method. In addition, this methodsimplifies detecting snaking car-trailer combinations in a curve.

During cornering, the yaw rate is given an offset so that theoscillation no longer swings about the zero point but about this offset.This fact renders detection more difficult. If, however, the differencebetween the model yaw rate and the measured yaw rate (yaw velocity) isused, this offset will be compensated. The detection signal will thusalways swing about zero.

Another especially favorable embodiment of the method provides that thedeviation between actual yaw rate and model yaw rate is additionallyweighted by a factor that is calculated in response to the model yawrate speed. The quicker the model yaw rate change is, the smaller thefactor becomes, which is, however, always >0. Said factor is multipliedby the differential value or differential value signal so that a lowdifferential value is the result in the event of a quick change of themodel yaw rate. Thus, detection is only allowed in the presence ofextreme oscillations, but is avoided in other cases. It is thereby takeninto account that with rapid steering movements the vehicle is no longerable to follow the vehicle model so that the difference between themodel yaw rate and the measured yaw rate shows a signal variation thatwould cause spurious detections.

In another especially favorable embodiment of the method, the number ofthe demanded half waves depends on the frequency of the oscillation. Themore half waves are demanded, the more reliable the detection ofspurious detections becomes. With low frequencies, however, it willpossibly last too long until an intervention can take place when greatnumbers of half waves are demanded. It is, therefore, favorable tointervene already at low frequencies when small numbers of half wavesprevail, yet to demand more half waves at high frequencies.

In another especially favorable embodiment of the method, the demandedoscillation amplitudes are speed-responsive. Oscillations are morecritical at high speeds than at low speeds. Therefore, detection takesplace already at low differential value oscillations when thecar-trailer combination runs at high speed, while the threshold israised at low speeds.

In still another especially favorable embodiment of the method, separateentry and exit thresholds are provided for the differential valueamplitudes. An intervention takes place only when the yaw rate exceedsthe high threshold. Thereafter, the intervention will only be terminatedwhen values drop below a lower exit threshold. This will ensure thatthere is a defined intervention and will prevent that the controller isconstantly activated and deactivated again.

As a second component of the detection, the method comprises a modulefor analyzing the lateral acceleration variation. Maximums and minimumsof the signal are determined. The frequency can be determined from thedistances in time between maximums and minimums. The frequency mustroughly correspond to the frequency of the differential value signal.The position of the maximums and minimums of the lateral accelerationsignal is compared with the position of the maximums and minimums of thedifferential value signal. The phase shift between differential valueand lateral acceleration can be calculated therefrom. The phase positionduring driving on rough roadways is different from the phase positionduring driving with snaking car-trailer combinations. The phase shift issmall with snaking car-trailer combinations. This criterion is examined,and the detection of a snaking car-trailer combination is forbidden inthe event of a too great phase shift.

In another especially favorable embodiment of the method, spuriouscontrol activations are prevented by way of several additionalplausibility tests of the lateral signals. The following signalvariations are untypical with snaking car-trailer combinations and,therefore, cause prevention or stop of interventions:

-   -   The frequency of the lateral signals is obviously changing        (becomes significantly lower or higher).    -   The frequency of the lateral signals lies outside the typical        frequency band.    -   The amplitude of the lateral signals is significantly        decreasing.    -   The difference of the maximums and minimums of the lateral        signal variations is small.    -   The absolute value of the average value of the lateral        acceleration is too high (extreme cornering maneuver; snaking        car-trailer combinations are not plausible in such maneuvers).

FIG. 4 shows a simplified view of the logical processes of the control:

Starting from the yaw rate difference 41 (Δ{dot over (ψ)}) between themodel yaw rate and the measured yaw rate determined in the ESP vehiclemodel (see e.g. the driving stability control according to FIGS. 1 and 2and their description in DE 195 15 056 which shall be part of thisapplication), the differential value 41 is filtered in step 40. Thismeans that the differential value 41 undergoes low-pass filtering sothat extreme peaks will not occur. Step 42 comprises the search for halfwaves in the input signal, which are analyzed by way of two zerocrossings, one maximum, a minimum amplitude and a defined initialgradient. It is polled in lozenge 43 whether the half wave was detected.If this is not the case, switch-back to step 42 is made and the searchfor half waves is continued. If the half wave was detected by way of theprevious criteria, it is checked in terms of its validity in lozenge 44.To this end, the following criteria are polled:

-   -   The maximum of the half wave must exceed a defined value.    -   The distance of the zero crossings (half wave length) must be in        the significant frequency range.    -   The hysteresis band must be left after a defined time.    -   Starting with the second wave found:        -   The half wave length must be identical with the previous            one.        -   The average value of the lateral acceleration must not be            higher than a defined value.        -   The lateral acceleration must have the same sign at the time            of the maximum of the half wave.        -   The lateral acceleration must have a half wave of roughly            the same duration.        -   The model yaw rate must have the same sign at the time of            the maximum of the half wave.        -   The model yaw rate must be smaller than the vehicle yaw rate            by a certain amount.

If all of these criteria are satisfied, the half wave is valid, and thehalf wave counter is incremented in step 45. In the case of asignificant amplitude decrease (current amplitude is only X% of theprevious amplitude), the counter will not be incremented but maintainsits value, what can lead to a later entry into the control. If not allthe criteria are satisfied, the half wave counter is reset to zero instep 48. It is found out in lozenge 46 whether N half waves aredetected. This will trigger a deceleration control of the vehicle instep 47.

The criteria allow a control during cornering and even during steeringmovements of the driver.

1-17. (canceled)
 18. A method for stabilizing a car-trailer combination,including a towing vehicle and a trailer moved by the towing vehicle,the method comprising: monitoring rolling motions of the towing vehicleincluding yaw velocity of the vehicle; detecting an actual or expectedunstable driving condition of the towing vehicle or the car-trailercombination; and performing measures to stabilize the driving condition,wherein the measures that stabilize driving are controlled dependent ona differential value that is produced from the monitored yaw velocityand a model-based yaw velocity and evaluated according to criteriaindicative of an unstable driving performance.
 19. The method accordingto claim 18 further comprising: determining a frequency and an amplitudeof each half wave of the differential value; comparing the determinedfrequency and amplitude with stored values; and evaluating the rollingmotion of the car-trailer combination dependent on the result of thecomparison.
 20. The method according to claim 19, the frequency isdetermined from zero crossings and a time between two zero crossings ofthe differential value.
 21. The method according to claim 19 furthercomprising: counting a number of the half waves of the differentialvalue, where the amplitude of each half wave reaches or exceeds athreshold value, and where each positive and negative half wave of thedetermined frequency lies within a band defined by a top and a bottomthreshold value; and initiating measures that stabilize driving when athreshold value representative of a number of half waves is reached orexceeded.
 22. The method according to claim 21, wherein the thresholdvalue representative of a number of half waves is determined independence on the frequency.
 23. The method according to claim 22,wherein at low frequencies, the threshold value is reached or exceededwith a smaller number of half waves than is the case at a highfrequency.
 24. The method according to claim 22, wherein the thresholdvalue of each half wave representative of the amplitude is determined atleast in dependence on quantities that represent the velocity of thetowing vehicle or the car-trailer combination or the trailer.
 25. Themethod according to claim 24, wherein with quantities describing a highspeed, the threshold value is reached or exceeded at lower amplitudesthan with quantities describing a low speed.
 26. The method according toclaim 21, wherein only a consecutive number of half waves of thedifferential value is counted, where the amplitude of each half wavereaches or exceeds an entry threshold value, and in that the measuresthat stabilize driving are terminated when values reach or fall belowonly one exit threshold value ranging below the entry threshold value.27. The method according to claim 18, wherein data is produced from avariation of the differential value.
 28. The method according to claim18, wherein the differential value is weighted with a value, which isproduced in dependence on a steering angle velocity or a steering angleacceleration or the model-based yaw rate.
 29. The method according toclaim 18, wherein lateral acceleration is detected and the variation ofthe lateral acceleration is evaluated according to criteria which allowchecking plausibility of the data obtained from the variation of thedifferential value and being assessed according to criteria indicativeof an unstable driving performance.
 30. The method according to claim29, wherein a maximum and minimum values of the lateral acceleration andtemporal distances of the maximum and minimum are determined, afrequency is determined and the determined frequency is compared withthe frequency of the differential value.
 31. The method according toclaim 29 further comprising: discontinuing the measures that stabilizedriving when at least one of the following conditions is satisfied: afrequency of a lateral signal, in particular the lateral acceleration,and/or the differential value reaches or exceeds or, respectively, fallsbelow a top or a bottom threshold value; a frequency of the lateralsignal changes in relation to the frequency of the differential valuetowards a top or a bottom limit value; an absolute value of an averagevalue of the lateral signal exceeds a threshold value. an amplitude ofthe lateral signal decreases with a high gradient; and a differencebetween the maximum and minimum values of the lateral signal lies in anarrow band.
 32. The method according to claim 29, wherein a phase shiftbetween the lateral acceleration and the differential value isdetermined and evaluated according to criteria that permit definingdriving situations.
 33. The method according to claim 32, wherein themeasures that stabilize driving are discontinued or the method isterminated, respectively, when a threshold value indicative of a greatphase shift is exceeded.
 34. A device for stabilizing a car-trailercombination, including a towing vehicle and a trailer moved by thetowing vehicle, wherein the towing vehicle is monitored in terms ofrolling motions and measures that stabilize driving are taken upon thedetection of an actual or expected unstable driving performance of thetowing vehicle or the car-trailer combination, the device comprising: adriving stability control having at least a yaw rate sensor for sensingthe yaw velocity and a vehicle model for producing a reference yawvelocity; a determining unit for determining a differential value fromthe yaw velocity and the reference yaw velocity; and a control unitcontrolling measures that stabilize driving dependent on data beingobtained from the variation of the differential value and evaluatedaccording to criteria indicative of an unstable driving performance.